Electrolytic stripping method

ABSTRACT

The present invention provides an electrolytic stripping method comprising electrolytically stripping: a copper plating film exposed from a silver plating film that partially covers a copper plating film formed on the entire surface of a member; and leaking silver formed on an exposed portion of the copper plating film and having a smaller thickness than the silver plating film, using the copper plating film as an anode, wherein the electrolytic stripping is carried out using a cyan compound-free electrolytic copper-stripping liquid, and the electrolytic stripping liquid comprises a compound capable of forming, with silver, a complex ion that is more easily dissociated than a complex ion of silver and cyan; and wherein silver and copper are deposited on a cathode which is used as a counter electrode to the anode and which is made of a metal that is chemically stable against the electrolytic stripping liquid.

FIELD OF THE INVENTION

The present invention relates to an electrolytic stripping method. Inmore detail, the invention relates to an electrolytic stripping methodof stripping: a copper plating film exposed from a silver plating filmthat partially covers a copper plating film formed on the entire surfaceof a member; and leaking silver formed on an exposed portion of thecopper plating film and having a smaller thickness than the silverplating film, by electrolytic stripping using the copper plating filmand leaking silver as an anode.

BACKGROUND OF THE INVENTION

In a lead frame to be used in semiconductor devices, as illustrated inFIG. 1, at a tip portion of each of inner leads 12 constructing a leadframe 10 made of an iron based alloy material such as an iron-nickelalloy (42 alloy) in the side of a die pad 14 (this tip portion will behereinafter referred simply as “tip portion”), a bonding portion 12 a tobe connected with one end portion of a gold wire, the other end portionof which is connected to a semiconductor element mounted on the die pad14, is formed.

On such a bonding portion 12 a, a silver plating film is formed, and theforegoing one end portion of the gold wire is connected to the bondingportion 12 a.

As illustrated in FIG. 1, in forming the die bonding portion 12 a havinga silver plating film formed thereon, at the tip portion of each of theinner leads 12 of the lead frame 10 made of an iron based alloy, acopper plating film is formed on the entire surrounding of the leadframe 10 by electrolytic copper plating.

As illustrated in FIG. 2A, the lead frame 10 having such a copperplating film formed thereon is put between rubber-made mask plates 18 aand 18 b in such a manner that only a copper plating film 16 of aportion for forming the bonding portion 12 a is exposed at a tip portion20 of the inner lead 12, and electrolytic silver plating for forming asilver plating film on the exposed copper plating film 16 is carriedout.

Next, as illustrated in FIG. 2B, after completion of the electrolyticsilver plating, when the mask plates 18 a and 18 b are removed, a silverplating film 22 is formed only at the portion for forming the bondingportion 12 a of the tip portion 20 of the inner lead 12.

Thereafter, the lead frame 10 is dipped in an electrolytic strippingliquid, and the copper plating film 16 which is exposed without beingcovered by the silver plating film 22 is removed by electrolyticstripping using the lead frame 10 as an anode. Thus, as illustrated inFIG. 2C, the bonding portion 12 a comprising the copper plating film 16and the silver plating film 22 is formed only at the portion for formingthe bonding portion 12 a of the tip portion 20 of the inner lead 12.

Now, in putting the lead frame 10 between the mask plates 18 a and 18 band forming a silver plating film on the exposed copper plating film 16,leaking silver 24 having a smaller thickness than the silver platingfilm 22 is also formed at a portion having covered by the mask plates 18a and 18 b as illustrated in FIG. 2B, due to leakage of an electrolyticsilver plating liquid because of strains of the mask plates 18 a and 18b, etc. This leaking silver 24 is also removed by electrolytic strippingof the copper plating film 16, and as illustrated in FIG. 2C, the silverplating film 22 is formed only at the portion for forming the bondingportion 12 a of the tip portion 20 of the inner lead 12.

For electrically stripping the leaking silver 24 and the copper platingfilm 16 without substantially stripping the silver plating film 22, anelectrolytic stripping liquid containing a cyan compound has been usedas described in the following Patent Document 1.

Patent Document 1: JP 59-031900 A (pages 2 to 3)

As described in Patent Document 1, by dipping the lead frame 10, onwhich the silver plating film 22 is formed only at the tip portion 20 ofthe inner lead 12, in an electrolytic stripping liquid containing a cyancompound and carrying out electrolytic stripping of copper using thislead frame 10 as an anode and a stainless steel plate as a cathode, itis possible to electrolytically strip the leaking silver 24 and thecopper plating film 16 without substantially stripping the silverplating film 22.

However, if the electrolytic stripping liquid containing a cyan compoundis continuously used, in electrolytically stripping the leaking silver24 and the copper plating film 16, a stripping rate of the copperplating film 16 and the leaking silver 24 is reduced within a relativelyshort period of time. For this reason, it is required to frequentlyexchange such an electrolytic stripping liquid.

Further, if it is intended to recover silver as a noble metal from thecyan compound-containing electrolytic stripping liquid to be exchangedand disposed, another special recover step is required.

SUMMARY OF THE INVENTION

Accordingly, an object of the invention is to provide an electrolyticstripping method using an electrolytic stripping liquid which is a cyancompound-free electrolytic stripping liquid and the life of which can begreatly extended as compared to that of cyan compound-electrolyticstripping liquids.

Other objects and effects of the invention will become apparent from thefollowing description.

To achieve the foregoing objects, the present inventors made extensiveinvestigations. As a result, electrolytic stripping was carried outusing, as an anode, the lead frame 10 in which the silver plating film22 is formed only at the tip portion 20 of the inner lead 12 asillustrated in FIG. 2B while using an electrolytic stripping liquidwhich is a cyan compound-free electrolytic stripping liquid containingtriammonium phosphate capable of forming a complex ion of silver andammonium that is more easily dissociated than a complex ion of silverand cyan. At this time, a cathode made of platinum was used as a counterelectrode to the lead frame 10 serving as the anode. As a result, it hasbeen found that silver and copper are deposited on the cathode whileelectrolytically stripping the leaking silver 24 and the copper platingfilm 16 without substantially stripping the silver plating film 22.

Further, it has been found that in this way, by carrying outelectrolytic stripping while depositing silver and copper on thecathode, the life of the electrolytic stripping liquid is prolonged ascompared to that of cyan compound-containing electrolytic strippingliquids, thus leading to accomplishment of the invention.

Specifically, the invention relates to an electrolytic stripping methodcomprising electrolytically stripping:

-   -   a copper plating film exposed from a silver plating film that        partially covers a copper plating film formed on the entire        surface of a member; and    -   leaking silver formed on an exposed portion of the copper        plating film and having a smaller thickness than the silver        plating film, using the copper plating film as an anode,    -   wherein the electrolytic stripping is carried out using a cyan        compound-free electrolytic copper-stripping liquid, and the        electrolytic stripping liquid comprises a compound capable of        forming, with silver, a complex ion that is more easily        dissociated than a complex ion of silver and cyan, and    -   wherein silver and copper are deposited on a cathode which is        used as a counter electrode to the anode and which is made of a        metal that is chemically stable against the electrolytic        stripping liquid.

In the invention, the compound capable of forming, with silver, acomplex ion that is more easily dissociated than a complex ion of silverand cyan is preferably a compound capable of forming, with silver, acomplex ion having a lower complex stability constant than the complexion of silver and cyan.

As this compound, at least one compound selected from the groupconsisting of ammonia water, an ammonium salt, a tartaric acid salt,phosphoric acid, and a citric acid salt can be used.

Also, since the electrolytic stripping liquid to be used in theinvention is an electrolytic copper-stripping liquid for electricallystripping a copper plating film, it is preferable that the electrolyticstripping liquid contains a copper compound or an aromatic nitrocompound as a copper-oxidizing agent and is adjusted so as to have a pHof from 9 to 12.

Here, the copper compound is preferably derived from an ammonium sourceand a copper source capable of forming a copper ammonium complex, addedto the electrolytic stripping liquid.

Further, by surrounding the cathode by a cathode bag, it is possible tocollect silver and copper particles, comprising silver and copperdeposited on the cathode, respectively, by the cathode bag; and bytaking out the cathode bag from the electrolytic stripping liquid, it ispossible to recover silver and copper comprising silver and copperdeposited on the cathode.

Incidentally, a lead frame made of an iron based alloy can be suitablyused as the member.

The reasons why the life of cyan compound-containing electrolyticstripping liquids which have been used so far is short may be consideredas follows.

Since a cyan ion in an electrolytic stripping liquid forms a stablecomplex ion with a silver ion in the electrolytic stripping liquid, asilver concentration in the electrolytic stripping liquid rises with anincrease of the electrolytic stripping treatment amount of leakingsilver. As the silver concentration in the electrolytic stripping liquidrises, the stripping rate of leaking silver and a copper plating film isreduced.

On the other hand, in the invention, the electrolytic stripping iscarried out using an electrolytic stripping liquid which is a cyancompound-free electrolytic copper-stripping liquid containing a compoundcapable of forming, with silver, a complex ion that is more easilydissociated than a complex ion of silver and cyan, while depositingsilver and copper on a cathode used as a counter electrode to a copperplating film and leaking silver serving as an anode.

As a result, in accordance with the electrolytic stripping methodaccording to the invention, it is possible to prevent a reduction in thestripping rate of the copper plating film and leaking silver caused byaccumulation of silver in the electrolytic stripping liquid; and it ispossible to greatly extend the life of the electrolytic stripping liquidas compared to conventional cyan compound-containing electrolyticstripping liquids.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial plan view to explain a lead frame as a member to besubjected to electrolytic stripping.

FIGS. 2A to 2C are partial cross-sectional views to explain partialsilver plating to be formed on a lead frame as shown in FIG. 1 andleaking silver.

FIG. 3 is a schematic view to explain one embodiment of an electrolyticstripping device.

The reference numerals used in the drawings denote the followings,respectively.

-   -   10: Lead frame    -   12: Inner lead    -   12 a: Bonding portion    -   14: Die pad    -   16: Copper plating film    -   22: Silver plating film    -   24: Leaking silver    -   30: Electrolytic stripping liquid    -   36: Direct current source    -   38: Cathode    -   40: Cathode bag    -   42: Circulation pump    -   44: Circulation line    -   46: Filter

DETAILED DESCRIPTION OF THE INVENTION

The electrolytic stripping liquid to be used in the invention is a cyancompound-free electrolytic copper-stripping liquid. As the electrolyticstripping solution, an electrolytic stripping liquid containing a coppercompound or an aromatic nitro compound as a copper-oxidizing agent canbe suitably used.

For the copper compound as the copper-oxidizing agent, a copper ammoniumcomplex is suitable. Such a copper ammonium complex can be formed byadding, to the electrolytic stripping liquid, ammonia water or anammonium salt as an ammonium source and copper sulfate, coppercarbonate, copper oxalate or copper hydroxide as a copper source.

The copper ammonium complex that is formed in the electrolytic strippingliquid having an ammonium source and a copper source added theretocomprises at least one of [Cu(NH₃)₂]²⁺, [Cu(NH₃)₄]²⁺, and [Cu(NH₃)₆]²⁺.

As the aromatic nitro compound, chloronitrobenzoic acid,2-chloro-4-nitrobenzoic acid, o-nitrobenzoic acid, m-nitrobenzoic acid,p-nitrobenzoic acid, ethyl p-nitro-benzoate, and sodium p-nitrobenzoatecan be suitably used.

An electrolytic stripping liquid which contains an aromatic nitrocompound as a copper-oxidizing agent but to which a cyan compound isadded cannot be expected to have a prolonged life.

The amount of the copper-oxidizing agent used in the electrolyticstripping liquid of the present invention is generally from 1 to 100g/liter, preferably from 5 to 40 g/liter.

Further, it is preferable that such an electrolytic stripping liquid isadjusted so as to have a pH of from 9 to 12. The pH adjustment may beachieved by using ammonia water as the ammonium source to be added as acompound capable of forming a copper ammonium complex, or using a pHadjustor such as sodium hydroxide.

Here, in the case where the pH of the electrolytic stripping liquid isless than 9, or in the case where the pH of the electrolytic strippingliquid exceeds 12, the stripping rate of the copper plating film tendsto be reduced.

A compound capable of forming, with silver, a complex ion that is moreeasily dissociated than a complex ion of silver and cyan is added to theelectrolytic copper-stripping liquid to be used in the invention.Examples of such a compound include compounds capable of forming, withsilver, a complex ion having a lower complex stability constant than thecomplex ion of silver and cyan. Specifically, one or more compoundsselected from the group consisting of ammonia water, an ammonium salt, atartaric acid salt, phosphoric acid, and a citric acid salt can be used.

Incidentally, ammonia water or an ammonium salt which is a compoundcapable of forming a complex ion with silver may be also used as anammonium source for forming a copper ammonium complex.

The amount of the compound capable of forming, with silver, a complexion that is more easily dissociated than a complex ion of silver andcyan, in the electrolytic stripping liquid is generally from 0.1 mg to100 g/liter.

The term “complex stability constant” as referred to herein means aconstant expressing the degree of dissociation of a complex ion. Lowervalues thereof mean that the formed complex ion is more instable andmore easily dissociated.

For example, the complex ion of silver and cyan is based on anequilibrium reaction between a silver ion and a cyan ion as shown in thefollowing formula 1, and the complex ion of silver and ammonium is basedon an equilibrium reaction between a silver ion and ammonia as shown inthe following formula 2.Ag⁺+2CN⁻<- >Ag(CN)₂ ⁻  Formula 1Ag⁺+2NH₃<->Ag(NH₃)₂ ⁺  Formula 2

In these equilibrium reactions, the stability constant (K_(CN)) as anindex expressing hardness of dissociation of the complex ion of silverand cyan can be expressed by the following equation 1, and its value(K_(CN)) is 1×10²¹.K_(CN)=[Ag(CN)₂ ⁻]/{[Ag⁺][CN⁻]²}  Equation 1

On the other hand, the stability constant (K_(NH3)) as an indexexpressing hardness of dissociation of the complex ion of silver andammonium can be expressed by the following equation 2, and its value(K_(NH3)) is 1.5×10⁷.K_(NH3)=[Ag(NH₃)₂ ⁺]/{[Ag⁺][NH₃]²}  Equation 2

In this way, since K_(CN) is larger than K_(NH3), the complex ion ofsilver and ammonium is easily dissociated, as compared to the complexion of silver and cyan, into a silver ion and ammonia.

By removing the silver ion in the electrolytic stripping liquid, thecomplex ion of silver and ammonium is more easily dissociated into asilver ion and ammonia.

The silver ion in the electrolytic stripping liquid can be easilyremoved by depositing silver on a cathode, which is a counter electrodeto the lead frame 10 serving as an anode, in carrying out electrolyticstripping with respect to the copper plating film 16 and leaking silver24 as illustrated in FIG. 2B. It is preferable that an electrode made ofa metal that is chemically stable against the electrolytic strippingliquid, such as platinum and stainless steel, is used as the cathode.

The electrolytic stripping using such an electrolytic liquid can becarried out using a device illustrated in FIG. 3. In the deviceillustrated in FIG. 3, the lead frame 10 made of an iron-nickel alloy(42 alloy) as illustrated in FIG. 1 is dipped as a member, in anelectrolytic stripping liquid 30 stored in an electrolytic strippingvessel 32 provided with a stirrer 34. On the lead frame 10, the copperplating film 16 is formed on the entire surface thereof by electrolyticcopper plating, and the bonding portion 12 a for connection is formed atthe tip portion of each of the inner leads 12. On the bonding portion 12a, the silver plating film 22 is formed, and the leaking silver 24 isalso present.

This lead frame 10 is connected to an anode of a direct current source36, and a cathode 38 made of platinum to be connected to a cathode ofthe direct current source 36 is dipped as a counter electrode of thelead frame 10 (the anode) in the electrolytic stripping liquid 30. Sucha cathode 38 is surrounded by a cathode bag 40. The cathode bag 40collects silver and copper particles comprising silver and copperdeposited on the cathode 38, respectively, and the like, and is made offibers having durability against the electrolytic stripping liquid 30,such as fibers made of PTFE (polytetrafluoroethylene) or PP(polypropylene) and formed into a bag shape.

The electrolytic stripping liquid 30 stored in the electrolyticstripping vessel 32 is circulated by a circulation pump 42, and a filter46 for separating particles floating in the electrolytic strippingliquid 30 and the like is provided in the way of a circulation line 44.

By passing a direct current between the lead frame 10 as the anode andthe cathode 38, each dipped in the electrolytic stripping liquid 30stored in the electrolytic stripping vessel 32 as illustrated in FIG. 3,from the direct current source 36, the copper plating film 16 exposed onthe lead frame 10 and the leaking silver 24 are stripped. The silver iongenerated in the electrolytic stripping liquid 30 by stripping or thelike of the leaking silver 24 does not form a stable complex ion such asa complex ion of silver and cyan. For this reason, the silver ion andthe copper ion in the electrolytic stripping liquid 30 become silver andcopper particles comprising silver and copper deposited on the cathode38, respectively, and the like and are collected by the cathode bag 40.Fine silver particles and the like which have passed through the cathodebag 40 are collected by the filter 46 provided in the circulation line44.

In this way, by separating the silver ion in the electrolytic strippingliquid 30 as silver particles comprising silver or the like, it ispossible to reduce the silver concentration in the electrolyticstripping liquid. For this reason, the method of the invention is freefrom the formation of a stable complex ion of cyan and silver in anelectrolytic stripping liquid as seen in the conventionally employedelectrolytic stripping using a cyan compound-containing electrolyticstripping liquid. Also, according to the method of the invention, it ispossible to prevent a reduction in the stripping rate of the copperplating film and the leaking silver due to accumulation of silver in theelectrolytic stripping liquid; and it is possible to greatly extend thelife of the electrolytic stripping liquid as compared to that ofconventional cyan compound-containing electrolytic stripping liquids.

In FIG. 3, though the metallic lead frame 10 is used as the anode, inthe case where a resin substrate is used, it is possible to carry outelectrolytic stripping of a copper plating film or the like by using acopper plating film formed by electroless plating on-the entire surfaceof the resin substrate.

EXAMPLES

The present invention will be illustrated in greater detail withreference to the following Examples, but the invention should not beconstrued as being limited thereto.

Example 1

After forming a copper plating film by electrolytic copper plating onthe entire surface of a lead frame made of an iron-nickel alloy (42alloy), a silver plating film was formed on the copper plating film byelectrolytic silver plating.

Next, using the device illustrated in FIG. 3 and using an electrolyticstripping liquid shown in the following Table 1, the copper plating filmand the silver plating film formed on the lead frame 10 as the anodewere continuously electrically stripped between the anode and thecathode 38 under conditions shown in the following Table 2. At the timewhen the treatment amount of silver reached 1,000 ppm, the silverconcentration in the electrolytic stripping liquid 30 stored in theelectrolytic stripping vessel 32 was measured. The results obtained arealso shown in Table 2. TABLE 1 Electrolytic stripping Triammoniumphosphate: 100 g/L liquid 1 Copper(II) hydroxide:  10 g/L 25% ammoniawater (for pH adjustment) pH: 10.0 Electrolytic stripping Ammoniumtartarate: 200 g/L liquid 2 p-Nitrobenzoic acid:  10 g/L pH:  9.5Electrolytic stripping Triammonium citrate: 150 g/L liquid 3 Copper(II)hydroxide:  5 g/L 25% ammonia water (for pH adjustment) pH:  9.8

TABLE 2 Electrolytic stripping condition Silver concentration Kind ofCurrent Bath in electrolytic electrolytic density temperature strippingliquid stripping liquid (A/dm²) (° C.) (ppm) Electrolytic 1 30 8stripping liquid 1 Electrolytic 0.5 50 7 stripping liquid 2 Electrolytic3 30 3 stripping liquid 3

In the electrolytic stripping using any of the electrolytic strippingliquids shown in Table 2, silver in the form of particles or the likewas collected by the cathode 38, the cathode 40 and the filter 46, andthe silver concentration in the electrolytic stripping liquid 30 waslow.

When the electrolytic stripping was carried out using the electrolyticstripping liquid 1, the silver concentration in the particles or thelike which adhered to the cathode 38, silver concentration in theparticles or the like which were collected by the cathode bag 40, andsilver concentration in the particles or the likw which were collectedby the filter 46 were measured. The results obtained are shown in Table3. This silver concentration was measured by dissolving the particles orthe like which adhered to the cathode 38 or which were collected by thecathode bag 40 or the filter 46 in 50% nitric acid. TABLE 3 Adhered toCollected by Collected by cathode 38 cathode 40 filter 46 Electrolytic200 ppm 9,700 ppm 70 ppm stripping liquid 1

Example 2

The copper plating film and the silver plating film formed on the leadframe 10 were continuously electrically stripped using the electrolyticstripping liquid 1 under the same current density and bath temperatureas those in the case of the electrolytic stripping liquid 1 of Example 1in the same manner as in Example 1, except that the cathode bag 40 wasremoved. When the treatment amount of silver reached 1,000 ppm, thesilver concentration in the electrolytic stripping liquid 30 stored inthe electrolytic stripping vessel 32, silver concentration in theparticles or the like which adhered to the cathode 38, and silverconcentration in the particles or the like which were collected by thefilter 46 were measured. The results obtained are shown in Table 4. Themeasurement of the silver concentration was carried out in the samemanner as in Example 1. TABLE 4 Silver concentration in electrolyticAdhered to Collected by stripping liquid cathode 38 filter 46Electrolytic 13 ppm 210 ppm 9,500 ppm stripping liquid 1

As is clear from Table 4, it is understood that likewise Example 1, thesilver concentration in the electrolytic stripping liquid 30 was low,and silver was collected by the cathode 38 and the filter 46.

Comparative Example 1

The copper plating film and the silver plating film formed on the leadframe 10 were continuously electrically stripped in the same manner asin Example 2 under the same current density and bath temperature asthose in the case of the electrolytic stripping liquid 1 of Example 1,except that a cyan based electrolytic stripping liquid (potassiumcyanide: 80 g/L, p-nitrobenzoic acid: 10 g/L, pH: 11) was used as theelectrolytic stripping liquid. When the treatment amount of silverreached 1,000 ppm, the silver concentration in the electrolyticstripping liquid 30 stored in the electrolytic stripping vessel 32,silver concentration in the particles or the like which adhered to thecathode 38, and silver concentration in the particles or the like whichwere collected by the filter 46 were measured. The results obtained areshown in Table 5. The measurement of the silver concentration wascarried out in the same manner as in Example 1. TABLE 5 Silverconcentration in electrolytic Adhered to Collected by stripping liquidcathode 38 filter 46 Cyan based 6,100 ppm 550 ppm 0 ppm electrolyticstripping liquid

As is clear from Table 5, in the cyan based electrolytic strippingliquid, though silver was somewhat deposited on the cathode 38, thesilver was not deposited to such extent that particles or the likecollectable by the filter 46 were formed. For this reason, the silverconcentration in the electrolytic stripping liquid 30 is high.

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

This application is based on Japanese patent application No. 2003-433701filed Dec. 26, 2003, the contents thereof being herein incorporated byreference.

1. An electrolytic stripping method comprising electrolytically stripping: a copper plating film exposed from a silver plating film that partially covers a copper plating film formed on the entire surface of a member; and leaking silver formed on an exposed portion of the copper plating film and having a smaller thickness than the silver plating film, using the copper plating film as an anode, wherein the electrolytic stripping is carried out using a cyan compound-free electrolytic copper-stripping liquid, and the electrolytic stripping liquid comprises a compound capable of forming, with silver, a complex ion that is more easily dissociated than a complex ion of silver and cyan; and wherein silver and copper are deposited on a cathode which is used as a counter electrode to the anode and which is made of a metal that is chemically stable against the electrolytic stripping liquid.
 2. The electrolytic stripping method according to claim 1, wherein the compound capable of forming, with silver, a complex ion that is more easily dissociated than a complex ion of silver and cyan is a compound capable of forming, with silver, a complex ion having a lower complex stability constant than the complex ion of silver and cyan.
 3. The electrolytic stripping method according to claim 1, wherein the compound capable of forming, with silver, a complex ion that is more easily dissociated than the complex ion of silver and cyan comprises at least one compound selected from the group consisting of ammonia water, an ammonium salt, a tartaric acid salt, phosphoric acid, and a citric acid salt.
 4. The electrolytic stripping method according to claim 1, wherein the electrolytic stripping liquid further comprises a copper compound or an aromatic nitro compound as a copper-oxidizing agent and has a pH of from 9 to
 12. 5. The electrolytic stripping method according to claim 4, wherein the copper compound is a copper ammonium complex derived from an ammonium source and a copper source.
 6. The electrolytic stripping method according to claim 1, wherein the cathode is surrounded by a cathode bag for collecting silver particles comprising the deposited silver.
 7. The electrolytic stripping method according to claim 1, wherein the member is a lead frame made of an iron based alloy.
 8. The electrolytic stripping method according to claim 1, further comprising circulating the electrolytic stripping liquid in the vicinity of the cathode through a circulation line equipped with a filter, thereby collecting silver particles comprising the deposited silver. 